1. Field of the Invention
This invention relates generally to a new composition of matter, a zinc oxide (ZnO) encapsulated fluorescent dye nanoparticle in about the 200 nm range, with the ZnO outer layer being about 10 nm. More specifically, this invention relates to use of the new nanoparticle material as a fluorescence probe, antibacterial agent, or cancer treatment. Also, this invention relates to ways to make the new material.
2. Background Art
The ongoing worldwide nanotechnology revolution is predicted to impact several areas of biomedical research and other science and engineering applications. Nanoparticle-assisted drug delivery, cell imaging and cancer therapy are important biomedical applications of nanotechnology. Development of core-shell nanostructures that combine multiple functions are of great interest for future nano-bio-technology and biomedical applications. For example, core-shell nanostructures containing a chemotherapeutic drug and a fluorescent dye could be used to release the drug at sites of interest while tracking the exact location of its delivery using imaging methods employing the fluorescence of the dye molecules. When organic dye molecules such as fluoroescein isothiocyanate (FITC) are exposed to harsh environments, they often suffer from freely interacting with solvent molecules, which can result in reduced performance of the dye. Encapsulation of the organic fluorescent dye in a core-shell nanostructure can not only add optical functionality, allowing the particles to be tracked and imaged easily, but can also enhance the stability and performance of the dye by protecting it from photobleaching and quenching from the background medium.
Several groups have employed fluorescent core-shell nanoparticles to add functional layers that can destroy disease causing cells, including cancerous cells. Mesoporous fluorescent silica particles developed by adding molecular sieve materials such as MCM-48 were used for site-oriented delivery of chemotherapeutic drugs and cell imaging. Recently, gold coated silica nanoparticles have been used to kill tumor cells via hyperthermia treatments. However, these treatment methods employing either the conventional chemotherapeutic drugs or hyperthermia suffer from lack of significant cell specificity. Both methods can kill normal cells along with cancer cells of interest.
In a recent work (K. M. Reddy, K. Feris, J. Bell, D. G. Wingett, C. Hanley, A. Punnoose, Appl. Phys. Lett. 90, 213902 (2007)), our group demonstrated the ability of identical ˜13 nm ZnO nanoparticles to kill bacterial cells at concentrations that are not toxic to human T lymphocytes. Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) bacteria were completely killed by ZnO nanoparticles at concentrations ≧3.4 mM and ≧1.0 mM, respectively, with minimal effects on primary human T cells. More recent experiments have shown that these nanoparticles have superior ability to kill both Jurkat and Hut-78 cancer cells at micromolar concentrations at which normal cells displayed no measurable cell death. Such an order of magnitude difference in toxicity to cancer cells relative to normal host T-cells makes ZnO nanoparticles a potential candidate for cancer therapy. Based on these interesting results, we have developed a core-shell structure with fluorescent FITC encapsulated in SiO2 as the core and nanoscale ZnO as a surface layer, hereafter referred as FITC-ZnO. These FITC-ZnO particles were synthesized using a novel one-pot methodology involving successive hydrolysis and condensation of FITC-linked silicate and zinc salt. The synergistic effects of ZnO and SiO2 nanostructures on the optical properties of the fluorophores and the toxic nature of the resulting nanostructures are explored in detail. Moreover, although reactive oxygen species generated at the surface of excited nanomaterials are generally thought to be responsible for observed biocidal effects, the detailed mechanism of toxicity and their selective nature to different biological systems still remains poorly understood. Adding fluorescence functionality to the ZnO nanostructures will aid future detailed in vitro and in vivo studies necessary to understand the fundamental interaction/uptake mechanism as well as improve our ability to evaluate and efficiently utilize the therapeutic potential of ZnO nanoparticles. The present work focuses on the preparation and characterization of fluorescent FITC-ZnO particles with an untouched outer ZnO nanolayer and their ability to selectively kill certain types of cancer cells and bacteria. However, no attempt is made in this work to address the mechanism of cell-particle interactions and toxicity. These will be the subject matter of future publications.
Semiconducting ZnO has good chemical stability, wide direct band-gap (Eg≈3.37 eV) and large excitation binding energy (60 meV), and has been studied for numerous applications including nanodevices, light-emitting diodes, sensors, luminescence, and photovoltaics. Thus the ZnO surface layer of these novel core-shell FITC-ZnO nanostructures has the added benefit of providing a robust platform for many future applications. For example, Dorfman et al. (A. Dorfman, N. Kumar, J. Hahm, Langmuir, 22, 4890 (2006); and A. Dorfman, N. Kumar, J. Hahm, Adv. Materials, 18, 2685 (2006)) developed nanoscale ZnO platforms for use as attractive substrates in fluorescence bioassays using FITC labels. They found that ZnO nanorod substrates could significantly enhance the fluorescence detection capability of proteins and nucleic acids without any need for amplification of detection signal. Moreover, ZnO nanocrystals/quantum dots might produce UV and visible fluorescence. Thus, addition of a stable surface layer of ZnO on the fluorescent FITC-SiO2 cores will provide the core-shell particles with two different fluorescence sources. Changes in the ZnO fluorescence when biological species are attached to the outer surface could provide opportunities for bio-sensing and the internal FITC fluorescence could serve as a reference standard to quantify relative change in intensity and the surface attached species. To the best of our knowledge, preparation and characterization of fluorescent dye encapsulated particles with an active ZnO finishing layer and their preferential cancer killing and bacterial inhibiting ability have not yet been reported.